Contrast problems with reflector

[spaceboy]

Hi everyone

I think I have a problem with either my Newts or my eyes. The sky can be dark to the eye but as soon as I look through the EP it's like the moon is half full and washing out my view. I have a helios 200p and a 250 SW skyliner dod. This issue was brought to my attension as I came from a Tal 200k with which there was never an issue. Stars are bright and the sky is black. I naturally put the washed out sky down to the change to reflectors that are faster F/5 & F/4.8 compaired to the slower F/10 of the tal. My 127Mak also gives black sky and reasonably bright stars. I have been informed on another thread that there should be no difference between F/5 and F/10 other than EP choice and magnification.

Can anyone help as I don't have a clue what I'm doing wrong. Most people recommend flocking the OTA to improve contrast so this is another reason why I thought this is just the way fast scopes are as I don't see so much the recommendation for SCT's or 200k's to be flocked.

Has anyone else had the same problem? As I wanted so much to get a larger aperture scope for DSO but I am really starting to regret getting rid of the tal to fund the dob

Any help would be appretiated as I worry views of smuged galaxies and nebula will be lost amongst the washed out views I'm getting. BTW so far Jupiter has been good through both but I don't notice the wash of the sky so much as Jupiter is so bright anyway.

[Astro_Baby]

Well you could try limiting the aperture (and effectively upping the F ratio) by using the small aperture mask on the scopes front dust cap. Thay have two round things - one of them is removable. Remove it (and hang it on the other one as a staorage space). That will increase the scopes focal ratio and may help........

BUT.....

And I seem to be alone in this - I find my F5 newt NEVER does good contrast when compared with my 4" refractor or my 4" Maksutov. Its more noticeable in the refractor though that contrast seems much higher.

Thats exactly what I would expect to be honest but others tell me its not so.

[John]

I had that issue, or something like that, when nearby lights from the neighbours and my own house get into the top end of the tube and wash out the contrast of all but the brightest objects. I found that a tall light shield around the top of the scope, projecting about 12" above the tube, helps a lot as the upper tube area, including the spider, secondary mirror and focuser, remains very dark now at almost all angles.

[Ags]

Hi, I saw this discussion on the other thread... are you making a comparison between the scopes at the same magnification? My understanding is background brightness increases with aperture and drops with magnification. If the scopes have different F ratios but have the same aperture and the same magnification (not the same eyepiece of course), then the sky background should be similar. not identical because different scopes suffer more from scattered light.

[SiriusB]

Bear in mind the exit pupil size at a given magnification will be a lot bigger for your reflectors than for the tal and Mak. Big exit pupils give brighter sky backgrounds.

Just as an example, from the backyard i find in my 80ed ,exit pupils greater than about 3mm make the sky background too bright most nights.

Only if i escape into the country can i use my really low powers effectively.

There's probably more too it than just that, but its worth bearing in mind.

As i understand it focal ratio plays no part in (visual) sky brightness- its down to your exit pupil & also anything reducing contrast in the optical train.

Someone correct me if i've got that wrong.

[spaceboy]

Right I will try to accurate with my reply but I'm no mathematician so the details will be vague.

Magnification... The tal had no issues regards a lighter back ground using plossl 40,32,25,20,17,15,10,9,6.5,4 this is all I had so no issues ever were found

The dob and 200p Has light back ground using almost all the above plossl other than 6.5 &4. Although no where near as bad but still noticeable to a certain degree TV's 20, 16 is better not been able to try the 9 and no issue found with 6mm Rad. So maybe 9 & 6 Tvs and maybe 6.5 & 4 plossl are upto the standard I had out of the tal. The plossl 4 & 6.5 are of no use at these mags in the dob for obvious tracking issues.

Barlows. I have seen improvement on 20mm if memory serves right?,17,15,10 plossl hence a previous thread seeing what peoples feedback was relating to this which didn't come back with much result besides it was only an option to save money on better eps which went out the window when I almost took the focuser out one time when I bashed into 6" ep/barlow combination hanging out me scope.

I have considered light shrouds to eliminate the secondary getting light washed. But I did not think about the little cap method but would this not waste the other 6" of scope??

Dose this all sound about right?

[spaceboy]

BUMP

This is all using the F/5 200p compaired to the TAL

[John]

.... But I did not think about the little cap method but would this not waste the other 6" of scope?? ...

Yes - it's effectively stopping the scope down to a much smaller aperture.

I'm at a bit of a loss to explain what's happening here

I have a 10" F/4.8 newtonian and, now that I use the light shield that I described in my earlier post, the background sky is more or less as dark as it is with my F/6.5 4" refractor.

The secondary and secondary support vanes of newtonians cause some light scatter but that would also have been the case with TAL 200K which has quite a large secondary obstruction as I recall.

Thats why I suggested stray light entering the top of the tube as a possible cause - the secondary and the eyepiece are up there on a newt wheras the eyepiece is at the bottom end of the tube with the TAL 200K. At least you could try a cardboard light shield and see it there is any improvement.

[Ags]

The Tal has twice the focal length of the 200p, so my understanding is that the contrast you see in the Tal with a 40mm eyepiece would be roughly the same as with a 20mm eyepiece in the 200p, not counting problems with light scatter and so on. But that's not what you are seeing.

[acey]

Poor contrast could be due to stray light, which could be due to inadequate baffling. Look through the focusser drawtube in daylight without an eyepiece and with the tube cap off. Ideally you want to see only the reflection of the primary mirror: anything else around that reflection is stray light reaching the eyepiece, which you want to minimise. Low-profile focussers are more vulnerable to this than higher ones. Take a look at this article:

http://www.urania.be/forum/download....e=baffling.pdf

I didn't have trouble with light sky background but did notice poor contrast on planets which I tackled using ideas from the article. I'd say that in my case baffling the focusser was probably the most important step.

http://stargazerslounge.com/equipment-discussion/99482-improving-flextube-contrast.html

[johninderby]

There is one other major cause of reduced contrast in a Newt or dob that most people forget about. Stray light entering the bottom of the OTA can wash across the surface of the primary mirror and cause as much of a problem if not more than stray light at the focuser end. Flocking around the inside of the bottom of the tube can really help.

A simple experiment is to cover the bottom end of your scope to seal out any light and see what happens.

John

[spaceboy]

Poor contrast could be due to stray light, which could be due to inadequate baffling. Look through the focuser drawtube in daylight without an eyepiece and with the tube cap off. Ideally you want to see only the reflection of the primary mirror: anything else around that reflection is stray light reaching the eyepiece, which you want to minimize. Low-profile focuser's are more vulnerable to this than higher ones. Take a look at this article:

http://www.urania.be/forum/download....e=baffling.pdf

I didn't have trouble with light sky background but did notice poor contrast on planets which I tackled using ideas from the article. I'd say that in my case baffling the focuser was probably the most important step.

http://stargazerslounge.com/equipment-discussion/99482-improving-flextube-contrast.html

Flocked the OTA, end of focuser flocked also and secondary mirror edge blackened. Haven't done baffling around the primary as I don't like messing near the mirrors if I can help it

Will try the cardboard tube and if it works get a good due shield. TBH my observatory is pretty well sheltered from stray light so it shouldn't be this.

I can't explain it you would need to experience it to understand where I'm coming from. The very 1st time I looked through the EP I had to look up as I thought someone had turned their security light on. This is when I looked at the sky to determine if it was properly dark. It was! but as soon as I look through the EP it's like there is a light on some where. It is actually a cool when you have no intension of looking at the stars as it's like a magicians trick or some thing.

BTW what would you say the lowest power (good contrast) ep to have in a 250 F/4.8 if high mags are required to get dark sky??

[sixela]

Hi everyone

I think I have a problem with either my Newts or my eyes. The sky can be dark to the eye but as soon as I look through the EP it's like the moon is half full and washing out my view.

If the sky is *any* brighter than with the naked eye then you have veiling glare caused by improper baffling.

Culprit #1: the focuser is too close to the front of the scope and light enters the focuser directly from the opposite end of the tube. Extend the tube opposite to the focuser with a light shield.

Culprit #2: light enters the tube from behind the primary.

Culprit #3: the tube wall opposite the focuser, behind the secondary, the secondary holder or the secondary's edge are not flocked (i.e. they're painted black but somewhat shiny).

Culprit #4: unblackened bolts, screws and washers you can see from the focal plane. I have a small bottle of Tamyia XF1 Flat Black just for things like that...

If it's "merely" as bright as with the naked eye, that's just the exit pupil that is as large as your fully dilated eye pupil, i.e. low magnification. Simply pick a smaller focal length eyepiece.

This issue was brought to my attension as I came from a Tal 200k with which there was never an issue.

But you NEVER use these scopes with very large exit pupils. So even if you do have veiling glare and improper baffling, it won't look as bad (but the TAL Klevtsov-Cassegrains are actually well baffled too).

I have been informed on another thread that there should be no difference between F/5 and F/10 other than EP choice and magnification.

In an f/5 system, pick an eyepiece that is twice as short as in the f/10. The sky background should be *exactly* as bright. Otherwise, there's a source of veiling glare on the scope where the background is lighter.

I don't see so much the recommendation for SCT's or 200k's to be flocked.

Baffling works differently on these scopes. The problem is that on a Newt you cannot baffle away some things, and they are still seen from their lit side (while on other scopes you can make baffles that are only seen from the shadow side from the focal plane).

But that doesn't mean you need flocking everywhere, just whatever you can see from the focal plane behind the secondary and whatever you can see around the primary in the secondary.

But before you flock, make sure you baffle correctly. I've seen people flock their tubes completely (including the irrelevant bits in the middle) but not make them long enough, and that's like closing a water tap on a sinking boat.

Has anyone else had the same problem?

Unfortunately, almost anyone who buys a short commercial Newtonian tube from Synta or GSO :{. That's why there are after-market light shields to extend the tube opposite the focusers (though it's not that hard to make your own).

[sixela]

I can't explain it you would need to experience it to understand where I'm coming from. The very 1st time I looked through the EP I had to look up as I thought someone had turned their security light on. This is when I looked at the sky to determine if it was properly dark. It was!

A lot of that, though, is simply an optical illusion: when you look up at the dark sky, there is no field stop to act as a reference for what is really black. Even at a very dark site, the sky really is not black at all, but the field stop of an eyepiece is!

Just to illustrate:

Squares A and B are the same shade of grey...

You don't notice it as often on f/10 scopes because if a 25mm eyepiece is your longest eyepiece and your pupil dilates to 7mm, the scope actually makes the sky background 7.8x darker than the naked eye one!

Very large exit pupils are very rarely the best ones for seeing faint objects. At darkish sites, even for large but faint objects you're often better of with exit pupils of 2-3mm (eyepieces in the 10-15mm range on an f/5 scope), and at very dark sites for very, very, very faint objects most people prefer the view at 4mm exit pupils (20mm eyepiece for an f/5 scope). There's a reason people with fast scopes all lust for a 21mm Ethos...

If you use even longer eyepieces, it's usually to give you more context (something which is simply inaccessible to a slower scope), but it won't actually show you more.

[pvaz]

I actually opened that pic on photoshop to check the color code for the squares... amazing how the eye can process contrast to cause such optical illusions.

[spaceboy]

I get it now! dam that took some effort

So how do you deal with low power EP's on reflectors or do you just use a cheap thing just to find objects then drop in the higher mag EP's to appreciate what your looking at? I take it anything over 21mm would not be worth spending big money on as the exit pupil is just getting wider and so my eye is taking in so much more light that it's all looking a wash?????

This was my argument OK fast scopes "DON'T SUCK IN LIGHT" I hold my hands up to being a dumb to that one, but I'm sure you can see where my confusion arose. My point is choosing between scopes is so much more a minefield than just "reflector or Refractor" for the beginner. This whole slow & fast issue I feel is not explained enough in the first place " I hold myself to example and I wouldn't say I'm new to astronomy"

I think the fact that I'm not a "beginner" and had my Tal for several years is the reason I was so determined to believe it was the fast scope that was the issue when actually it was the mentality I had developed to what eyepieces I most feel comfortable viewing with that was causing the issue. I think the problem has arose as I read in magazines that you can have an exit pupil up to 8mm in youths so I set myself at 6.5mm when choosing EP's. Only upon my new found wisdom I have found the most my eyes have been used to from the TAL is 4.0mm with the 40mm EP which TBH rarely got used. So my favored 25=2.5mm, 20=2.0, 10=1.0 & 6.5 approx 0.6 would be dimmer to my eye compared to using the same EP's in the 200p 25=5.0, 20=4.0, 10=2.0 &6.5 approx 1.2.

I would have never dreamed of putting a 4mm in my Tal but I gather this would be acceptable in the 200p 250x mag 0.8 exit pupil. I often used near these magnifications in the TAL 9mm 222x with good results.

Have I grasped everyones explanation correctly??? As what I'm saying makes sense to me know. My eyes have never experienced the kind of light from an eyepiece and it would also explain why the 3-6 zoom Nags sell within 2 minutes of being listed!

THANK YOU ALL SO MUCH FOR YOUR HELP AND THANKS MOST TO SIXELA AS YOU MAY HAVE JUST SAVED ME A LOT OF MONEY! :icon_salut:

[sixela]

So how do you deal with low power EP's on reflectors or do you just use a cheap thing just to find objects then drop in the higher mag EP's to appreciate what your looking at?

Essentially, yes (of course, moving to a darker sky also helps). That's what makes the Ethos and Explore Scientific 100° AFOV eyepieces so popular on fast scopes: you can get closer to optimal magnification and still see a large field of view.

Another thing you can do is use more aggressive filtering. When I use my 21mm Ethos, I usually use an Omega Optical GCE or deepsky filter even when looking at galaxies and clusters, and obviously OIII/UHC filters when looking at emission nebulae. These make the sky background dimmer (the object too, but less than the sky background, and you usually have more surface brightness than at higher powers anyway).

But of course, some objects simply demand a large FOV. Even on small fast scopes, if you want to see all of M31 at once, you need a huge field and long widefields, and even things like M42, M44, M45, the North America nebula, both sides of the Veil nebula at once, the Rosette nebula etc. require quite a bit of field of view. At least with a fast scope (and a 2" focuser) you *can* increase the field of view at the expense of slightly lower perceived contrast; on a slow scope you simply have to switch scopes.

I take it anything over 21mm would not be worth spending big money on as the exit pupil is just getting wider and so my eye is taking in so much more light that it's all looking a wash?????

Well, it's not really looking more of "a wash": the sky background becomes lighter but so does the object, and you do get used to teasing detail out of the view despite the relatively high sky background.

For extended objects, the ratio of (surface brightness background+object)/(surface brightness of background) doesn't change, but the object becomes smaller you'll indeed see less and less detail, and if you have too much light pollution your eye also doesn't adapt to the dark as well when you use this magnification as with prolonged eyepiece time when using more magnification.

For stars, yes, you see less and less of them in any given piece of sky (the stars themselves don't become any brighter with lower magnification, but that lower magnification makes the background brighter.)

I think the fact that I'm not a "beginner" and had my Tal for several years is the reason I was so determined to believe it was the fast scope that was the issue

Well, a TAL is well built and well baffled. Better baffled than most mass-market Newtonians, but you can fix that (if necessary) fairly easily.

[spaceboy]

Thanks again Sixela.

I read much talk about the veil and I would love to see this in my own scope but I take it this would be a big money eyepiece and filter for me to acheive? As you have explained the low power so well where would I stand with the higher power EP choices as I assume these are more important as they offer the better contrast My main objective is galaxies, nebula and globular clusters. I have binoculars for larger M31 and open clusters.

Thanks

SPACEBOY

[John]

....I read much talk about the veil and I would love to see this in my own scope but I take it this would be a big money eyepiece and filter for me to acheive? ....

A normal 32mm plossl and a UHC or O-III filter will do it.

Edit: oh, and dark skies !.

[spaceboy]

Thanks John They don't do a 2" plossl do they :iamwithstupid:just lately I feel I need to check. My head is spinning with every thing I taken in the last couple of days.

I think Sixela already covered hi power didn't he??? TMB planetary?? is the FOV wide enough on these for dob nudging??

[John]

They used to do 2" plossls but nowadays they seem to have been replaced with wide angle designs that seem to be derived from the Erfle.

The TMB / TS Planetaries have a FoV of 58 degrees so not to bad for dob nudging.

[spaceboy]

I need to be careful when choosing a cheap low power ep that I don't end up with other view problems caused with fast scope. Will a plossl be OK or would it be better to get one of those erfle ep's Antares 32mm 2" Modified Erfle Eyepiece

[John]

You have a 20mm Nagler according to your signature - that will show as much sky as a 32mm wide angle. I'd not noticed that you had the Nagler before - that would be brilliant for DSO's.

[spaceboy]

Will this show all the veil nebula though?

[John]

Will this show all the veil nebula though?

Yes, of course

You will still need the filter and dark skies though - Naglers arn't magic